Recovery of carbon dioxide from gas mixture

ABSTRACT

Carbon dioxide is recovered from gas mixtures containing it in a concentration not greater than the equilibrium concentration at the freezing temperature of the mixture, by subjecting the gas mixture to membrane separation and recovering from the membrane separation a permeate having a carbon dioxide concentration between the equilibrium concentration and 98% (by volume), distilling said permeate at subambient temperature above the freezing temperature of the permeate and recovering substantially pure carbon dioxide as a liquid bottoms product of the distillation. Carbon dioxide is recovered from gas mixtures containing it in a high concentration by supplying the gas mixture at superatmospheric pressure and distilling said gas mixture at sub-ambient temperature in a distillation column, recovering substantially pure carbon dioxide as a liquid bottoms product of the distillation, warming the overhead stream from the distillation to a temperature suitable for membrane separation, passing the overhead stream at superatmospheric pressure over a membrane which is selectively permeable for carbon dioxide and thereafter recycling carbon dioxide-rich permeate for recompression and feeding to the distillation column.

This is a continuation-in-part of parent, copending application Ser. No.675,292, filed Nov. 17, 1984, and now abandoned in favor of the presentapplication.

This invention relates to the recovery of high purity carbon dioxidefrom gas mixtures. In a first embodiment the invention is particularlyconcerned with the treatment of gas mixtures in which the carbon dioxideis present in an amount below the equilibrium concentration at thefreezing point of the mixture. In a second embodiment the invention isconcerned with the treatment of gas mixtures in which the carbon dioxideis present in high concentrations.

Processes for the recovery, purification and liquefaction of carbondioxide from carbon dioxide rich streams e.g. fermentation gas and offgases from chemical processes such as ethylene oxide production orammonia production, are well known. These processes normally carry outthe final purification (involving removal of light gases such ashydrogen, nitrogen, oxygen, methane and carbon monoxide) either bycooling and partial condensation of the gas, thus producing a liquidenriched in carbon dioxide and a tail gas lean in carbon dioxide, or bycooling and distillation if a purer carbon dioxide product e.g. greaterthan 99% pure, by volume, is required.

Such processes are limited by the fact that gas mixtures containingcarbon dioxide have a relatively high freezing point. Pure carbondioxide freezes at a temperature of 216.4° K. and while this temperaturemay change depending on the other components in the mixture and thepressure, it is still the factor which determines the degree ofseparation which can be achieved.

If a gas mixture contains less than the equilibrium concentration ofcarbon dioxide at the freezing temperature of the mixture concerned thenthe carbon dioxide cannot be separated by cooling and partialcondensation or by cooling and distillation, since the carbon dioxidewill freeze before any liquid is formed. This is illustrated in FIG. 1which shows the dewpoint curves for nitrogen/carbon dioxide mixtures,and FIG. 2 which shows a typical temperature composition diagram fornitrogen/carbon dioxide mixtures at various pressures. At a pressure of20 bar, for example, the equilibrium carbon dioxide content of thevapour at the freezing temperature of 216.5° K. is approximately 30% byvolume. This means that at a pressure of 20 bar, gaseous mixtures ofnitrogen/carbon dioxide which contain less than approximately 30% byvolume carbon dioxide cannot be separated by cooling followed by partialcondensation or distillation.

Many gases exist where the carbon dioxide content of the gas is low andpurification directly by cooling and partial condensation cannot beachieved directly due to freezing problems, e.g. lime kiln gas, boilerflue gas and certain natural gases.

A solution to this problem, which is used commercially, is to scrub thegas mixture which is lean in carbon dioxide with a suitable solvent,e.g. monoethanolamine, sulfolane or potassium carbonate, whereby todissolve the carbon dioxide and then to strip the carbon dioxide fromthe solution so obtained; i.e. another fluid is introduced into thesystem in order to achieve the necessary separation. The carbon dioxidecan then be compressed, dried, cooled and further purified by partialcondensation or distillation. However this process is expensive inenergy and a less energy-intensive alternative would be desirable.

In a process developed for the recovery of high purity carbon dioxidefrom a gas mixture, the gas mixture is compressed, preferably to apressure of approximately 15 to 25 bar, and dried by absorption or otherconventional means (all pressures are given in bar absolute). Afterremoval of undesirable impurities such as sulphur containing compounds,the mixture is cooled and separated by distillation, yielding highpurity carbon dioxide as a bottoms product. However, the overheadproduct of the distillation column will always contain a significiantproportion of carbon dioxide, generally at least 60 percent, which isnormally wasted.

The high percentage of carbon dioxide in the overhead stream is due tothe equilibrium conditions prevailing in mixtures with lighthydrocarbons which, at the usual operating pressures of around 20 barand condenser temperatures around -30° C., lead to vapour concentrationsaround 60 to 70% CO₂. This situation is not substantially improved byoperation at lower temperatures, which may lead to azeotrope formationand/or solidification of carbon dioxide. As a result, particularly whenthe feed gas contains a significant amount of non-condensibles orinerts, such as nitrogen, oxygen or methane, it is not possible toapproach 100% recovery of carbon dioxide. For example with only 3 molpercent of non-condensibles or inerts present, the CO₂ recovery would belimited to about 95 percent.

A gas separation method that has recently found commercial success isthe membrane separation process which involves passing the mixture to beseparated at superatmospheric pressure over a semi-permeable membraneacross which a pressure drop is maintained and through which one or moreof the components of the gas mixture is selectively permeable. Thistechnique has already been proposed for th removal of carbon dioxide asan undesirable impurity from a gas stream, and also for the separationof carbon dioxide from an oil well stream for recycle to the oil well toassist recovery of oil. In neither case, however, is the purity of theseparated carbon dioxide stream a matter of importance, nor is highrecovery achievable.

Although it would be possible theoretically to recover carbon dioxide ina high degree of purity, e.g. 99% or more pure, from a gas mixture bymembrane separation, this would be uneconomical because of the highpressure and/or number of separation steps that would be required.

It has now been found unexpectedly that carbon dioxide can be recoveredeconomically and at a high level of purity, e.g. 99% by volume or purer,from a gas mixture which contains the carbon dioxide in a concentrationat or below the equilibrium concentration at the freezing point of themixture, by combination of membrane separation and distillation undercertain conditions.

It has also been found unexpectedly that the recovery of high purity CO₂(e.g. 99% by volume or purer) from gas mixtures in which a highconcentration of carbon dioxide is present can be raised to aneconomically desirable level, e.g. 99 percent or more by employing in aparticular manner a combination of distillation and membrane separationusing a suitable membrane.

According to one aspect of the present invention, there is provided aprocess for the recovery of carbon dioxide from a gas mixture containingit in a concentration not greater than the equilibrium concentration atthe freezing temperature of the mixture, the process comprisingconcentrating the carbon dioxide in the mixture to a level between saidequilibrium concentration and 98% (by volume) of the mixture by membraneseparation and thereafter distilling the concentrate at sub-ambienttemperature and recovering carbon dioxide as a liquid bottoms product ofthe distillation.

While the process can be operated satisfactorily with the carbon dioxidecontent of the gas (concentrate) recovered from the membrane separationas low as 40% or 50%, by volume, the best results from an economicalpoint of view are obtained when the gas contains at least 85% and morepreferably at least 90% carbon dioxide, by volume. Carbon dioxide canthen be recovered at levels of purity similar to those achievable by theknown process of solvent extraction and distillation but with an energysaving which can be as much as 20% or even more in some cases,especially if the overhead stream (i.e. tail gas) from the distillationstep is recycled to the membrane separation step.

While the membrane separation may be operated in a single step, it ispreferred to employ at least two steps with the carbon dioxide-enrichedpermeate of the first being subjected to a second separation, ifnecessary with intermediate recompression. While more than two steps maybe employed, if desired, the further improvement so obtained is not sogreat. Where the membrane separation involves more than one separationstep, any recycle of the overhead stream from the distillation columnmay be to the first or a subsequent step of the membrane separations, asdesired. In some cases, such recycle may be desirable not only tooptimise carbon dioxide recovery but also to recover one or more othercomponents of the feed gas stream which are retained in said overheadstream from the distillation column.

The process is particularly applicable to the treatment of gases whichcontain carbon dioxide in a mixture with nitrogen and/or methaneoptionally together with other hydrocarbons, e.g. ethane, propane and/orbutane.

According to another aspect of the present invention there is provided aprocess for the recovery of carbon dioxide from a gas mixture containingit in a high concentration, the process comprising supplying the gasmixture at superatmospheric pressure and substantially free ofcontaminants which would solidify under the process conditions,distilling said gas mixture at sub-ambient temperature in a distillationcolumn and recovering substantially pure carbon dioxide as a liquidbottoms product of the distillation, warming the overhead stream fromthe distillation to a temperature suitable for membrane separation,passing the overhead stream at superatmospheric pressure over a membranewhich is selectively permeable for carbon dioxide and thereafterrecycling carbon dioxide-rich permeate for recompression and feeding tothe distillation column.

By a temperature suitable for membrane separation, is meant atemperature at which the physical properties of the membrane are suchthat it is mechanically stable and a sufficient fluid flux can bemaintained across the membrane. Thus the temperature should not be solow that the membrane would be embrittled and consequently unable towithstand the pressure differential across it, nor so high that themembrane would be softened and distort to an unacceptable extent.Suitably the overhead stream is warmed to a temperature in the range 0°to 50° C., preferably 0° to 30° C. and most preferably to around ambienttemperature.

In a preferred embodiment, the overhead stream from the distillationcolumn is warmed by indirect heat exchange with a refrigerant employedin closed cycle to provide cold for the column reflux. In suchcircumstances design features of the refrigeration cycle and anappropriate choice of refrigerant can advantageously lead to theoverhead stream being warmed to ambient temperature.

The process of the invention enables the achievement of a high recoveryof high purity carbon dioxide from gas streams containing carbon dioxidein high concentrations. For example a recovery of above about 90% andmost preferably above about 95% carbon dioxide can be achieved. Undersuitable conditions the recovery of carbon dioxide can be improved to99% or more of the CO₂ in the feed gas. For gas streams containing lowerconcentrations of CO₂, e.g. below about 40%, separation is usuallycarried out for the purpose of removing the CO₂ as an undesirableimpurity, rather than for recovering CO₂. Examples of gas streamscontaining carbon dioxide in high concentrations from which it may bedesired to separate carbon dioxide are certain natural gases and gaseffluents from petrochemical works and other gas sweetening operations.In such cases the associated constituents are mainly light hydrocarbonssuch as methane and ethane.

Any suitable membrane may be employed but it is preferred to use thosewherein the carbon dioxide permeability is at least 10 times that of thegas or gases from which it is to be separated under the chosenseparation conditions. Examples of suitable membranes are those formedfrom polysulphone or cellulose acetate.

The invention is now described in more detail with reference topreferred embodiments and with the aid of the accompanying drawings inwhich

FIG. 3 is a flow diagram of the process for recovery of carbon dioxidefrom a gas mixture containing it in a concentration not greater than theequilibrium concentration; and

FIG. 4 is a flow diagram of the process for recovery of carbon dioxidefrom a gas mixture containing it at a high concentration.

Referring to FIGS. 3, 1 and 5 are membrane separation units, 2 and 6 arecompressors, 3 and 7 are coolers, 4 and 8 are gas/liquid separators, 9is an adsorber unit, 10 is a distillation column, 11 is a reboiler, 12is a condenser and 13 is a refrigeration unit.

The carbon dioxide-containing feed gas mixture, at a suitably elevatedpressure, is provided through pipeline 20 to membrane separation unit 1,where it is separated into a carbon dioxide-rich permeate which isrecovered through line 22 and a tail gas which is lean in carbon dioxideand which is recovered through line 24. The gas in line 22 is recoveredfrom membrane unit 1 at a pressure below the desired inlet pressure tomembrane unit 5. It is first recompressed in compressor 2 and thencooled in cooler 3, any condensate thereby formed being separated ingas/liquid separator 4 and the resultant gas being supplied to membraneunit 5 through pipeline 26.

In membrane unit 5, further carbon dioxide-enrichment occurs with thecarbon dioxide-rich stream being recovered through pipeline 28 and thelean gas stream being recovered through pipeline 30.

The gas in pipeline 28 is then supplied to distillation column 10. Ifthe pressure at which it is recovered from the membrane separation unit5 is below the desired distillation pressure, the gas is firstrecompressed in compressor 6, cooled in cooler 7 and any condensate soformed is separated in gas/liquid separator 8. Further drying can becarried out, if desired, by passing the gas through adsorber unit 9. Thegas is then cooled to the desired distillation temperature by passagethrough distillation column reboiler 11 and then fed to an intermediatepoint in the distillation column. High purity carbon dioxide liquid,e.g. 99% or more pure, is recovered from the bottom of the column inpipeline 32 and the tail gas is recovered in pipeline 34 and returned tobe combined with the feed gas in pipeline 26 to the second membrane unit5. In the embodiment illustrated, the cooling for the condenser of thedistillation column is provided by a vapour compression refrigerationunit 13 but other cooling means may also be used.

EXAMPLE 1

Using the process described above with reference to FIG. 3 of thedrawings, the recovery of a substantially 100% carbon dioxide liquidstream from a gas stream comprising 88 mole% methane and 12 mole% carbondioxide and supplied at a pressure of 28.6 bar and a temperature of 300°K. required an energy consumption of 1936 Kcals/Nm³ of liquid carbondioxide produced.

The details of temperature, pressure and composition of the gas streamsin the various parts of the process are set out in Table 1 below.

                  TABLE 1                                                         ______________________________________                                        Pipe- Flow     Composition       P                                            line  (Nm.sup.3 /h)                                                                          Mole % CH.sub.4                                                                          Mole % CO.sub.2                                                                        (Bar) T° K.                         ______________________________________                                        20    100 000  88.0       12.0     28.6  300                                  24    82 226   97.0        3.0     28.0  310                                  22    17 774   46.4       53.6      1.5  303                                  26    19 530   46.4       53.6     27.5  303                                  28    10 158    8.0       92.0      1.5  300                                  32     8 402   --         100.0    28.0  266                                  34     1 756   46.4       53.6     27.5  240                                  30     9 372   88.0       12.0     27.0  303                                  ______________________________________                                    

The energy requirements expressed as Kcals/Nm³ of liquid CO₂ produced,were as follows, assuming 30% efficiency for gas engine drives:

    ______________________________________                                        Compressor 2:       958                                                       Compressor 6:       548                                                       Refrigeration Unit: 410                                                       Carbon Dioxide Dryer 9:                                                                            20                                                       Total               1936                                                      ______________________________________                                    

By way of comparison, the energy requirements for treating the samestream by the known solvent extraction process employing Sulfinol are2401 Kcals/Nm³ of liquid carbon dioxide produced and the energyrequirements for treating the same stream by a conventional solventextraction process employing a mixture of mono- and di-ethanolamine are4379 Kcals/Nm³ of liquid carbon dioxide produced.

Referring to FIG. 4, 41 is a compressor, 42 is an after cooler, 43 is apurification unit of conventional kind, 44 is a drier, 45 is adistillation column, 46 is a reboiler, 47 is a reflux condenser, 48 is arefrigerant compressor, 49 is a refrigerant condenser, 50 is a heatexchanger, and 51 is a refrigerant expansion valve, and 52 is a membraneseparation unit.

The carbon dioxide-containing feed gas mixture is provided throughpipeline 60 usually at a pressure in the range 0.7 to 2.0 bar,compressed in compressor 41 usually to a pressure in the range 10 to 30bar, and then cooled in after cooler 42 to remove the heat ofcompression. The gas mixture is then supplied via line 61 topurification unit 43 where sulphur-containing impurities are removed byknown means such as absorption or catalytic reaction. When such asulphur removal step is carried out at high temperature in a catalystbed, then this may precede the after-cooler 42. After purification thegas mixture is passed through line 62 to drier 44 which is ofconventional form using beds of absorbent. The dried gas mixture is thensupplied via line 64 to distillation reboiler 46 where it gives up heatto provide reboil for the distillation and is cooled to the desireddistillation temperature usually -3° to -40° C. The thus-cooled feed isthen fed to an intermediate point in the distillation column, whichgenerally operates with a bottom temperature in the range -40° to -12°C. and an overhead temperature in the range -50° to -20° C. Thedistillation generally operates at a pressure which is slightly, e.g. 1to 3 bar, below the pressure of gas leaving compressor 41, e.g. apressure in the range 10 to 25 bar. High purity carbon dioxide e.g. 99%or more pure, is recovered from the bottom of the column in line 66 andthe tail gas is recovered in line 67 warmed in heat exchanger 50,usually to a temperature of 0° to 50° C. and fed to membrane separationunit 52. There the tail gas is separated into a carbon dioxide-richpermeate which is recycled via line 72 to feed line 60 and a streamdepleted in carbon dioxide which is recovered through line 70. Usingthis process a high recovery of high purity carbon dioxide can beobtained.

In the embodiment illustrated in FIG. 4 the cold for production ofliquid CO₂ and for the distillation column reflux is provided by aclosed regrigeration cycle for which in this case the refrigerant isammonia. In the refrigeration cycle, warm liquid refrigerant from thecondenser, 49 passes via line 76 to exchanger 50 where it is sub-cooledbefore expanding through valve 51 and passing via line 74 to refluxcondenser 47, where it is evaporated by indirect heat exchange with thecarbon dioxide containing gas mixture. The refrigerant is thencompressed by refrigerant compressor 48, cooled and condensed inrefrigerant condenser 49.

The tail gas from the distillation must be warmed before contact withthe membrane separation unit.

It is suitably warmed to ambient temperature, that is to a temperaturein the range 0° to 50° C., preferably 0° to 30° C., and most preferablyambient temperature, to avoid potential problems associated withmechanical instability of the membrane at low temperatures. It isconvenient to do this by heat exchange with a condensed refrigerant asdescribed above in which refrigerant condensation has conveniently beeneffected by heat exchange with air or cooling water provided at ambienttemperature. Another possibility is to warm the tail gas by heatexchange with the carbon dioxide containing gas mixture before saidmixture is fed to the reboiler 46. In either case the tail gas willpreferably be warmed to about ambient temperature before contact withthe membrane separation unit.

EXAMPLE 2

The details of temperature, pressure and composition of the gas streamsin various parts of the process described above with reference to thedrawing are set out in Tables 2 and 3 below. The carbon dioxide recoveryin this case is 99.6%.

                  TABLE 2                                                         ______________________________________                                        Line          Temp                                                            (°C.)  (bar)   Pressure                                                ______________________________________                                        60            +30     1.0                                                     61            +30     17                                                      62            +30     16.6                                                    64            +35     16.3                                                    66            -29     14.5                                                    67            -35     14.3                                                    68            +20     14.2                                                    72            +15     1.1                                                     74            -40     0.72                                                    76            +40     15.5                                                    ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Flows in kg mol/hr (dry basis)                                                Component   CH.sub.4                                                                             C.sub.2 H.sub.6                                                                         CO.sub.2                                                                            Total                                      ______________________________________                                        Line                                                                          60          0.12   1.73      152.53                                                                              154.38                                     64          0.13   1.82      157.53                                                                              159.48                                     66          --     --        151.94                                                                              151.94                                     68          0.13   1.82       5.59  7.54                                      70          0.12   1.73       0.59  2.44                                      72          0.01   0.09       5.00  5.10                                      ______________________________________                                    

What is claimed is:
 1. A process for the recovery of carbon dioxide froma gas mixture containing carbon dioxide in a concentration not greaterthan the equilibrium concentration at the freezing temperature of themixture, the process comprising subjecting the gas mixture to membraneseparation;recovering from said membrane separation a permeate whereinthe concentration of carbon dioxide is between said equilibriumconcentration and 98% (by volume) of the permeate and thereaftersubjecting said permeate to distillation at sub-ambient temperatureabove the freezing temperature of the permeate; and recoveringsubstantially pure carbon dioxide as a liquid bottoms products of saiddistillation.
 2. A process as claimed in claim 1 in which the carbondioxide content of the permeate obtained by membrane separation is atleast 85% by volume.
 3. A process as claimed in claim 1 in which thecarbon dioxide content of the permeate obtained by membrane separationis at least 90% by volume.
 4. A process as claimed in claim 1 in whichthe membrane separation step involves the use of at least two membranesin series.
 5. A process in accordance with claim 4 in which the gasrecovered from one of said at least two membranes is repressurised priorto feeding to the next of said membranes.
 6. A process as claimed inclaim 1 in which the overhead stream from the distillation is recycledto the first or a subsequent membrane of the membrane separation step.7. A process as claimed in claim 1 in which the gas mixture alsocontains nitrogen.
 8. A process as claimed in claim 1 in which the gasmixture also contains at least one hydrocarbon.
 9. A process as claimedin claim 8 in which the gas mixture also contains methane.
 10. A processfor the recovery of carbon dioxide from a gas mixture containing it in ahigh concentration, the process comprising supplying the gas mixture atsuperatmospheric pressure and substantially free of contaminants whichwould solidify under the process conditions, distilling said gas mixtureat sub-ambient temperature in a distillation column and recoveringsubstantially pure carbon dioxide as a liquid bottoms product of thedistillation, warming the overhead stream from the distillation to atemperature suitable for membrane separation, passing the overheadstream at superatmospheric pressure over a membrane which is selectivelypermeable for carbon dioxide and thereafter recycling carbondioxide-rich permeate for recompression and feeding to the distillationcolumn.
 11. A process as claimed in claim 10 in which the overheadstream from the distillation column is warmed to a temperature in therange 0° to 50° C. before being fed to said membrane.
 12. A process asclaimed in claim 10 in which the overhead stream from the distillationcolumn is warmed to a temperature in the range 0° to 30° C. before beingfed to said membrane.
 13. A process as claimed in claim 10 in which theoverhead stream from the distillation column is warmed before being fedto said membrane by indirect heat exchange with a refrigerant employedin closed cycle to provide cold for the column reflux.
 14. A process asclaimed in claim 10 in which the carbon dioxide is recovered from a gasmixture containing it in a concentration greater than 40%.
 15. A processas claimed in claim 10 in which more than 90% by volume of the carbondioxide in the gas mixture is recovered as a liquid bottoms product ofthe distillation.
 16. A process as claimed in claim 10 in which morethan 95% by volume of the carbon dioxide in the gas mixture is recoveredas a liquid bottoms product of the distillation.